Hermansky-Pudlak Syndrome (HPS) is a heritable multisystem disorder in which tissue-specific, lysosome-related subcellular organelles in certain cell types are improperly formed. Mutations in any of 6 genes in humans and an additional 10 genes in mice cause HPS or an HPS-like syndrome. For the products of most of these genes, a function is not known. Defining the functional role of these gene products is a key to combating the disease. Among the symptoms of HPS is oculocutaneous albinism due to malformation of melanosomes, lysosome-related organelles in eye pigment epithelia and melanocytes of the skin and eye in which melanin pigments are made and stored. Melanosomes develop through a series of stages characterized by distinct morphologies and cargo, each of which are distinguishable from conventional endosomes and lysosomes that coexist with melanosomes in human pigment cells. Nevertheless, there is an intimate relationship between melanosomes and endosomes, such that complex, tissue-specific sorting pathways are required to segregate cargo destined for different compartments. Much of this sorting occurs in multivesicular compartments. We hypothesize that the cellular machinery responsible for segregating cargo includes proteins that regulate sorting at multivesicular endosomes in all cell types as well as the products of HPS-associated genes. We further hypothesize that the HPS-associated gene products, particularly HPS3 and subunits of the BLOC-1 complex, functionally interact with more general endosomal factors in regulating tissue-specific transport steps involved in melanosome biogenesis. Defining these steps will allow us to understand how unique tissue-specific organelles are formed and will help to develop therapies for HPS patients. The specific aims are: 1. To test the hypothesis that sorting of the stage I/II melanosome marker, Pmel17, relies on novel sorting determinants for sequestration within multivesicular endosomes. 2. To test the hypothesis that effectors of endosomal multivesicular body (MVB) formation regulate discrete stages of melanosome formation. 3. To test the hypothesis that HPS-associated gene products function in protein sorting at the stage I melanosome.